Means for preventing hum due to interelement capacities in vacuum tubes



June 7, 1932. w. 1,. KRAHL MEANS FOR PREVENTING HUM DUE TO INTERELEMENT CAPACITIES IN VACUUM TUBES Filed Dec. 7, 19 28 \NPUT' OUT PUT 25 uunnm OUTPUT 6/ noun.

Z INVENTOR 5 M47R laws KR/IHL ATTORNEY Patented June 7, 1932 UNITED STATES PATENT lorries WALTER LOUIS KRAHL, OF MONTCLAIR, NEW JERSEY, ASSIGNOR TO ARCTURUS RADIO TUBE COMPANY, OF NEW'ARK, NEW JERSEY, .NCOBPGRATION OF DELAWARE MEANS FOR PREVENTING HUM DUE TO INTERELEMENT CAPACITIES IN VACUUM TUBES Application filed December 7, 1928. Serial No. 324,517.

m ternating currents flowing through the grid circuit of the detector tube will produce variations upon the grid which, when magnified by the detector tube and the amplifying tubes of the radio set, will produce varia- 15 tions in the output voltage of the set to cause ,i! in; such potential drop being transferred to Inasmuch as capacities between the elea hum in the loud speaker. The grid leak,

due to its high resistance, causes considerable drop 1n potential in the grid clrcuit when even mlnute alternating currents flow therethe grid and this alternating current flowing through the grid circuit may be due to the capacity between the grid and the heater circuit of the tube.

.5 It is therefore one of the objects of the ments of different tubes vary somewhat, it is also another object of the invention to provide a means in a radio set which may be adjusted to compensate forthis interelement 3 capacity in any tube which may be inserted in the set.

Another object of the invention is to prevent any fluctuating current from flowing through the grid leak of a detector tube energized by fluctuating current. I

Other objects of the invention will be apparent as the description-thereof proceeds.

The invention is illustrated in the accompanying drawing, in which Fig. 1 is a curve showing the variable output voltage of a'direct current radio set with respect to the capacity which is in series with the grid and a source of alternating current of a predetermined voltage;

Fig. 2 is a circuit diagram of a four terand minal alternating current tube showing the manner of connecting the compensating means in the circuit; 1

Fig. 3 is a diagrammatic interpretation of the figure shown in Fig. 2;

Fig. 4 is a circuit diagram of a five terminal tube showing the manner of connecting pm compensating means in the circuit there or; s Fig. 5 is a diagrammatic interpretation of the circuit shown in Fig. 4; and

Fig. 6 is a circuit diagram of a. modification of Fig.5.

With the use of a very small capacity con-V denser connected inseries with the secondary of a transformer across which a (SO-cycle potential of volts was maintained and also connected in series to the input'circuit of a direct current detector tube to the output of which an amplifier was connected, I was able to ascertain the voltage variation produced in the output circuit by the alternating current passing through the small capacity condenser. The usual condenser id leak were connected between the grid and the cathode of the detector tube, so that any current passing through the small capacity condenser could complete the cir cuit through the grid leak.

The results of these experiments were,

somewhat astounding, as will be evident from an inspection of Fig. 1. From almost zero capacity the voltage curve rises sharply to about two and one-half volts when the capacity of the condenser is only one micromicrofarad, and to about sixteen volts when the capacity of the condenser is about five micromicrofarads. An output voltage variation of two volts with the average radio amplifier now in use will produce an audible hum in the loud speaker, but inasmuch as it is extremely difficult to define what is an audible hum, as such a sound will depend entirely upon the sensitiveness of the amplifier and of the speaker, as well as of the ear of the listener, I desire to use the word hum as meaning any voltage variation in the output circuit caused by the alternating heater potential.

In certain types of alternating current tubes the cathode is connected to the heater circuit inside of the tube so that the tube has four terminals. Such a tube is illustrated in Fig. 2, in which a plate 10 is shown connected to a terminal 11; a grid 12 is shown connected to a terminal 13; a cathode 14 is shown connected to a terminal 15; and a heater 16 is shown with its lower end connected to a terminal 17 and with its upper end also connected to the cathode terminal 15. The terminal 11 is connected through a B battery 18 to an output terminal 19 in the circuit, and the grid terminal 13 is connected through a grid leak 20 with its usual by-pass condenser 21 to an input terminal 22, which forms the usual connection for a detector tube. The terminal 23 forms the other terminal for both the input and output, as indicated, and is connected to the cathode terminal 15. Thus both the input and output circuits are connected to the cathode.

It will be evident that the upper end of the heater is connected through the input transformer and the grid leak to the grid, and inasmuch as there is no resistance in any part of this circuit carrying alternating cur rent, there is no drop of potential and hence, the capacity between the wires connected to the terminal 15 and the grid of the tube produces no disadvantageous result and need not be considered. However, it will also be evident from an inspection of the drawing that between the grid and the lower end oi the filament which is connected to the terminal 17 following the metallic circuit through the input, there is interposed the entire re sistance of the heater. Thus the circuit extends from the grid terminal 13 through the grid leak 20, terminal 22, through the input circuit, terminal 23, heater cathode terminal 15, through the hi h resistance heater to the terminal 17. Inasmuch as the resistance of the heater is in this circuit, it will be evident that there will be a difierence in alternating current potential between the terminal 17 and associated wires, and the grid terminal 13 and its associated wires. It, then, there is a capacity, as represented by the condenser 24 shown in dotted line, between these two terminals 13 and 17, an alternating current will flow therebetween. Stated in another way the input, grid leak and capacity 24 are connected in series directly across the heater leads and a current will flow in this circuit J depending on the size of the capacity 24 and the voltage across the heater leads. This current flowing through the grid leak causes a voltage drop and produces a variation on the grid which, as can be readily seen, will be amplified by the remaining tubes in the circuit to produce a variable voltage in the output of the set.

In order to determine just what the capacity was between the grid terminal of an alternating current tube and the terminal connected to the lower end of the heater, I carefully measured these capacities in a number of tubes and found that it is about 0.4 of a micromicrofarad. I also measured a standard four terminal tube socket and found that the capacity between the grid and this heater terminal, which in this case is the adjacent heater terminal, is about .65 micromicrofarads. It will be evident that the sum of these two capacities, which represents the capacity when the tube is in the socket, will be 1.05 micromicrofarads, and from an inspection of the curve in Fig. 1, it will be seen that this capacity when subject to a pressure of fifteen volts, will produce a voltage variation in the output circuit of nearly three volts which, as has been stated above, will produce a hum in the speaker..

The invention is designed to compensate for the capacity between the grid and the lower end of the heater, and to this end I have introduced a resistance 25, one end of which is connected to the terminal 15 and the other end of which is connected to one side 26 of the power supply line at terminal 27 The secondary 28 of the transformer 29 is connected to the wire 26, and the other side of the secondary is connected by means of wire 30 to the terminal17, thus placing the secondary 28 in series with the resistance 25 and across the heater terminals 15 and 17. A primary winding 31 connected to a source of alternating current 32 furnishes the power supply, as will be evident, to energize the heater and cathode of the tube, A variable condenser 33 is also connected between the terminal 13 and the terminal 27.

With the addition of the resistance 25 and the variable condenser 33, the resistances and capacities of the circuit may be represented as indicated at Fig. 3. This circuit, as will be seen, takes the form of a WVheatstonc bridge with the terminals 13, 17, 15 and 27 as the four terminals of the bridge. The heater 16 and additional resistance 25 form the resistance on one leg of the bridge, and the capacity 24 between the terminals 13 and 17 and the variable condenser 33 form the other leg, With the grid leak 20 and the grid condenser 21 forming the diagonal. If a fixed resistance is inserted at 25, it will be clearly evident that the variable condenser 33 may be adjusted to such a point that no current will flow through the grid leak 20, and this adjustment will entirely balance out the capacity effect between the grid and lower end of the heater so that no alternating current whatever will flow in the grid leak circuit, and, hence, no hum will be reproduced.

It will be understood that, if desired, the condenser 33 may be made fixed and the resistance 25 may bevaried until a point is reached giving the same result, or this result may be attained by varying one or more of the other elements of the bridge.

If desired, the by-pass condenser21 may be connected to the terminal 27 and utilized as the balancing capacity. This is advantageous, since it eliminates the condenser 33 which affects the tuning of the input circuit.

lVith tubes having five terminals in which the cathode is-not connected at all to the heater circuit within the tube, there is a somewhat difierent result set up by the capacities between the grid and the ends of the heater. The combined base and socket capacity for the grid and heater terminals of this type of tube is approximately .68 micromicrofarads, which, it will be seen from reference again to Fig. 1, may produce a hum in the output of the set. A circuit connection embodying a five ter minal tube is shown in Fig. 4 in which the plate 34 is shown connected to terminal 35;-

the grid 36 to terminal 37; the cathode 38 to terminal 39; the lower end of the heater 40 to terminal 41; and the upper end of the heater to terminal 42. The plate terminal 35 is connected to the B battery 43; the grid terminal 37 is connected to the grid leak 44 and the grid condenser 45 and the negative side of the B battery, and the other end of the grid leak are connected respectively to terminals 46 and 47, forming one terminal each of the output and the input for the circuit. The other terminal 48 of the output and the other terminal 49 of the input are connected together and to the lead 50, which is connected to the cathode terminal 39. The secondary 51 of the transformer 52, having a primary 53 connected across a source 54 of alternating current energy, is connected by means of the wires 55 and 56 to terminals 41 and 42, respectively, which are the heater terminals of the set. The wire 50 connecting the terminals 48 and 49 to the cathode terminal 39 isalso connected to the arm of potentiometer 57 which is connected across the heater leads 55 and 56.

Where a five terminal tube is used, as in the present instance, and the cathode is entirely independent from the heater circuit, some means is necessary to make a connection between the cathode and the heater'circuit. This has been accomplished by the use of the potentiometer 57, the grid and plate returns of the circuit being connected to the arm 58 which contacts at some point spaced from the ends of the coil. Thus'the grid terminal 37 is connected electrically to the terminal 41 through a portion of the potentiometer coil, and the terminal 42 is connected to the grid terminal through the other por tion of the potentiometer coil. It is evident that the grid is thus connected to a point having an alternating potential with relation to both terminals of the heater, and that a small capacity 59 exists between the grid and one'terminal of the heater, and a small capacity 60 exists between the grid and the other terminal of the heater.

. The various'parts of the circuit shown in F ig.v 4 may be represented by the diagram shown in Fig. 5, wherein the terminal 37 and the arm 58 of the potentiometer are shown at opposite ends of the grid leak 44 and condenser 45, which form the diagonal of a lNheatstone bridge, and terminals 41 and 42 are shown indicating the other two corners of the bridge circuit. Power is suppliedat the terminals 41 and 42 from the source 54. The capacity between terminals 41 and 37 is represented by the condenser 59, and that between the terminals 42 and 37 is represented by the condenser 60. The arm of the potentiometer connects at a point on the coil thereof, forming two resistances-6l and 62, the former between the arm of the potentiometer and the terminal 41 and the latter between the arm and the terminal 42. It will be evident from an inspection of this diagram that by shifting the arm 58 of the potentiometer a point may be reached upon the coil where no current will flow through the grid leak 44, and when this'point is-found the capacities betwen the ends of the heater and the grid will be compensated for.

While a potentiometer has. been used in.

this place before, attention is drawn to the fact that the sole purpose of this potentiometer has been to connect the grid and plate return circuits to a point having the potential of the midpoint of the filament, or a point which divides the filament into two sections from which the electronic emission is equal. It is to be noted that my invention does more than this, as it finds a point whereby the circuit is balanced so that the interelement capacities are compensated for and no alternating current flows through the grid leak. This point is oftentimes not at the center of the filament or at a point which divides the filament into two sections having equal emission. If merely the latter results are obtained, there may still be a hum in the output circuit arising from the interelement capacities described above.

Besides the capacity which exists between the grid and heater terminals and the wires associated therewith, there isalso a conductance across the surface of the glass seal which although very slight, aids the current flow in the circuit, and it will be evident that the effect of this conductance may also be elimi-: nated by the invention.

Certain modifications may be made in the invention, such as changing the elements of the VVheatstone bridge circuits as shown in Figs. 3 and 5, without departing from the spirit of the invention, and I desire, therefore, to interpret the invention broadly withinthe scope of the appended claims.

Having thus described my invention, what I desire to claim is:

1. In a radio set, a vacuum tube, plate and grid elements within said'tube, a cathode within said tube, a circuit associated with said cathode for energizing said cathode by fluctuating current, an output circuit connected between'said cathode and said plate, an input circuit connected between said cathode and said grid and including agrid leak, a connection between said output and input circuits and said energizing circuit, and means including a resistance preventing fluctuating current from flowing through said grid leak due to the capacity or conductance between said grid and associated circuit and said fluctuating current circuit.

2. In a radio set, a vacuum tube, plate and grid elements within said tube, a cathode within said tube, a separate heater for said cathode, a circuit connected to said heater for energizing said heater with fluctuating current, an output circuit connected between said cathode and said plate, an input circuit connected between said cathode and said grid and including a grid leak, a connection between said output and input circuits and said energizing circuit, and means including a resistance preventing fluctuating current from flowing through said grid leak due to the capacity or conductance between said grid and associated circuit and said fluctuating current circuit.

3. In a radio set, a vacuum tube, plate and grid elements within said tube, a cathode within said tube, a circuit associated with said cathode for energizing said cathode by fluctuating current, an output circuit connected between said cathode and said plate, an input circuit connected between said cathode and said grid and including a grid leak, connection between said input and output circuits and said energizing circuit, a capacity between said grid and one side of said energizing circuit and between said grid and the other side ofsaid energizing circuit, and means preventing fluctuating current from flowing through said grid leak due to said capacities between said grid and energizing circuit.

4. In a radio set, a *acuum tube, plate and grid elements within said tube, acathode within said tube, a circuit associated with said cathode for energizing said cathode by fluctuating current, an output circuit connected between said cathode and said plate,

Ian input circuit connected between said cathode and said grid and including a grid leak, a connection between said input and output circuits and said energizing circuit, a capacity between said grid and one side of said energizing circuit and between said grid and the other side of said energizing circuit, and a balancing resistance interposed between one side of said energizing circuit and the connection of said input and output circuits thereof.

5. In aradio set, a vacuum tube, plate and grid elements within said tube, a cathode within said tube, a separate heater for said cathode, a circuit connected to said heater for energizing said heater with fluctuating current, an output circuit connected between said cathode and said plate, an input circuit connected between said cathode and said grid and including a grid leak, a connection between said output and input circuits and said energizing circuit, a capacity between said grid and one side of said energizing circuit and between said grid and the other side of said energizing circuit, and means preventing fluctuating current from flowing through said grid leak due to the said capacities between said grid and energizing circuit.

6. In a radio set, a vacuum tube, plate and grid elements within said tube, a cathode within said tube, a separate heater for said cathode, a circuit connected to said heater for energizing said heater with fluctuating current, an output circuit connected between said cathode and said plate, an input circuit connected between said cathode and said grid and including a grid leak, a connection between said output and input circuits and said energizing circuit, a capacity between said grid and one side of said heater energizing circuit and between said grid and the other side of said heater energizing circuit, and a balancing resistance interposed between one side of said energizing circuit and the connection of said input and output circuits thereof.

7. In a radio set, a five terminal vacuum tube, plate and grid elements within said tube, a cathode within said tube, a heater for said cathode entirely separate therefrom, said heater connected to two of said terminals and said cathode connected to a third terminal, a circuit connected to said heater terminals for energizing said heater with fluctuating current, an output circuit connected to said plate, an input circuit connected to said grid, meansfor compensating for the conductance path of fluctuating current between the grid terminal and both terminals of the heater circuit and their associated circuits comprising a resistance across said heater circuit, and a connection from said input and output circuits to a point on said resistance at which the circuits are so balanced that no fluctuating current will flow through said grid leak.

8. In a radio set, a four terminal vacuum tube, plate and grid elements within said tube, a cathode within said tube, a separate heater for said cathode, a circuit connected to said heater for energizing said heater with fluctuating current, an output circuit connected between said cathode and said plate, an input circuit connected between said cathode and said grid and including a grid leak, a connection between said cathode and one end of said heater within said tube, and means including a resistance to prevent fluctuating current from flowingthrough said grid leak due to the capacity or conductance between said grid and associated circuit and said fluctuating current circuit.

9. In a radio set, a four terminal vacuum tube, plate and grid elements within said tube, a cathode within said tube, a separate heater for said cathode, a circuit connected to said heater for energizing said heater with fluctuating current, an output circuit connected between said cathode and said plate, an input circuit connected between said cathode and said grid and including a grid leak, a connection between said cathode and one end of said heater within said tube, and means to prevent fluctuating current from flowing through said grid leak due to the capacity or conductance between said grid and associated circuit and said fluctuating current circuit, said means including a resistance in series with the end of the heater having the cathode connected thereto, and a variable condenser between the grid and the extreme end of said resistance.

10. In a radio set, a four terminal vacuum tube, plate and grid elements within said tube, a cathode within said tube, a separate heater for said cathode, a circuit connected to said heater for energizing said heater with fluctuating current, an output circuit connected between said cathode and said plate, an input circuit connected between said cathode and said grid and including a grid leak, a connection between said cathode and one end of said heater within said tube, and means to prevent fluctuating current from flowing through said grid leak due to the capacity or conductance between said grid and associated circuit and said fluctuating current circuit, said means including a resistance in series with the end of the heater, having the cathode connected thereto, and a condenser between the grid and the extreme end of said resistance, one of said last mentioned elements being variable.

In testimony whereof, I aflix m signature.

WALTER LOUIS RAI-IL. 

